1. Field of the Invention
This invention pertains to the art of color filters and more specifically to an energy efficient color filter for producing illumination of a selected color in a lamp.
2. Description of the Prior Art
Conventional incandescent lamps for producing light of a particular color, for example, red, blue or green, are generally of several known types. The first uses a so called absorptive filter in which the desired color is produced by filters placed external to the lamp or by a finish applied directly to the lamp envelope, usually on the outside. The filters have an absorptive action, that is, they absorb light energy in the unwanted portion of the spectrum which is transformed into heat for reradiation. Energy of the desired wave length (color) is transmitted through the filter. These types of filters generally are of the organic type, e.g. paints, or possibly a silicon coating.
Another type of lamp for producing a selective color utilizes a multi-layer filter coating of a number of non-metallic films of low and high refractive indicies which are vaporized onto the glass envelope. Each layer of the coating is one quarter wave length thick, resulting in high reflectances at a particular wave length. Combinations of these layers of materials and their thicknesses produce a desired spectral distribution of transmitted light. In general, such coatings are called "dichroic filters" and have as many as 15 to 21 layers. Such lamps are disclosed, for example, in an article by Beesley entitled "New High Efficiency Color for
Lamps Using Multi Layer Interference Coatings" appearing in Ilumination Engineering March 1964 (pages 208-212).
Another type of color filter is described in U.S. Pat. No. 4,366,407 assigned to the assignee of the present invention and utilizes an "etalon" coating in which a thin film layer of an insulating material is located between two thin film layers of a metal, the coating being called a composite metal-insulator-metal coating. The thin films of the coating are formed on the wall of the incandescent lamp envelope with the thickness of the individual films of the coating and their interrelationship selected so as to maximize the coating transmission characterisitics to energy produced by the filament for a wavelength of a particular color in the visible range. Also, the color filter coating can be formed to maximize the reflecting properties to energy not desired to be transmitted including that in the infrared range, and in conjunction with an optically shaped envelope, reflect this energy back to the lamp filament to increase the lamp's efficiency.